In an internal memo announcing his departure, Irion apologised to colleagues for the "abruptness" of the news. "So now two years into the acquisition it's time for me to get started on some long brewing ambitions," he adds.

Irion headed Dutko's energy and environment division, before being named president of the firm in 1996. He helped build the firm into one of DC's top five lobbying shops, with a particular focus on integrating advocacy, government relations and public affairs.

Before joining Dutko, Irion worked as a legislative assistant and Senate aide, focusing on environmental and energy issues.

"We all worked too hard from where we began to become a top 10 firm, and then a top five firm for six years running to not keep building," adds Dutko in the memo. "Whatever challenges lay in front to reclaim that position are nothing compared to what it took us to get there."